Fix non-ASCII key handling in wxGrid editors.

Fix non-ASCII key handling in wxGrid editors.

Use wxKeyEvent::GetUnicodeKey() correctly, there is no need to guess about
what does it return now that its correct behaviour is documented and
implemented. Simply check if it returns WXK_NONE to check for non-characters.

Also use WXK_START instead of hard-coded 255 when checking GetKeyCode()
result.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65748 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775